Diffusion furnace



Nov. 5, 1968 G. P. HAMPTON DIFFUS ION FURNACE INVENTOR. Gordon P Hampton BY 1%, Wp'ias,

Attorneys x C C mm NV Nov. 5, 1968 G. P. HAMPTON DIFFUSION FURNACE 5 Sheets-Sheet Filed June 21, 1967 INVENTOR. Gordon R Hampton BY 7%, W,Mw

Attorneys Nov. 5, 1968 G. P. HAMPTON 3,409,727

DIFFUSION FURNACE Filed June 21, 1967 5 Sheets-Sheet 3 III I I f I I I I I I I INVENTOK Gordon P Hampton BY flwm W m 9L W Attorneys United States Patent 3,409,727 DIFFUSION FURNACE Gordon P. Hampton, Sunnyvale, Calif., assignor to Electroglas Inc., Menlo Park, Califi, a corporation of California Filed June 21, 1967, Ser. No. 647,826 12 Claims. (Cl. 13-20) ABSTRACT OF THE DISCLOSURE Diffusion furnace having a substantially hollow processing member with a processing zone therein with a helical heating element surrounding the processing member. Means is provided for supporting the heating element which includes a tube and a plurality of circumferentially spaced comb-like members carried by the tube and having teeth which are disposed between the loops of the helical heating element to support the heating element within the tube.

Background of the invention This invention relates to diffusion furnaces. Diffusion furnaces have heretofore been provided as disclosed in U.S. Letters Patent 3,299,196, in which the coils of a helical heating element have been separated by individual spacers in which the heating element rests inside a concentric supporting tube.

Summary of the invention The diffusion furnace consists of a substantially hollow processing member which has a processing zone therein. A helical heating element formed of a plurality of loops with spaces therebetween surrounds the processing member. Means is provided for Supporting the heating element and includes a tube concentric with the heating element and a plurality of circumferentially spaced comb-like members disposed within the tube. The comb-like members are provided with spaced teeth which are disposed in the spaces between the loops to maintain the spacing between loops and also to support the heating element so that it is spaced from the concentric tube.

In general, it is an object of the invention to provide a diifusion furnace which can be more economically manufactured and which particularly reduces labor costs.

Another object of the invention is to provide a diffusion furnace in which the desired operating temperature can be reached more quickly.

Another object of the invention is to provide a dilfusion furnace of the above character in which the heating element is spaced from the concentric tube surrounding the same and is supported therein so that it is spaced from the concentric tube.

Brief description of the drawings FIGURE 1 is a cross-sectional view showing :a portion of a difiusion furnace incorporating the present invention.

FIGURE 2 is a cross-sectional view taken along the line 22 of FIGURE 1.

FIGURE 3 is a cross-sectional view taken along the line 3-3 of FIGURE 2.

FIGURE 4 is a cross-sectional view taken along the line 4-4 of FIGURE 1.

FIGURE 5 is an enlarged view of the portion of the diffusion furnace encircled by the line 5--5 shown in FIGURE 4.

FIGURE '6 is :a cross-sectional view looking along the line 6-6 of FIGURE 5.

FIGURE 7 is a cross-sectional view of another embodiment of a diffusion furnace incorporating the present invention.

3,409,727 Patented Nov. 5, 1968 Description of the preferred embodiments As described in U.S. Letters Patent 3,299,196, a diffusion furnace typically consists of a large control cabinet (not shown) upon which is mounted a plurality of furnace assemblies 11, only one of which is shown in FIG- URE 1. Each of the furnace assemblies 11 consists of a rectangular metal casing 12. A processing or diffusion member 13 in the form of a cylindrical tube is disposed within the casing 12. The processing member 13 is formed of a suitable material such as quartz. It is typically provided with one open end 14 and :a necked down end portion 16. The processing member 13 encloses a processing zone 17 which is used for diffusion purposes as hereinafter described.

A helical heating element 19 extends longitudinally of the processing member 13 and surrounds the processing member 13. The helical heating element is provided with a plurality of spaced loops or coils 21 which have spaces 22 between the same. The heating element 19 can be of any desired cross-section, circular, for example, as shown in the drawings; or rectangular, if desired. The processing member 13 merely rests within the helical heating element 19 and can be removed therefrom or inserted therein as desired.

Means is provided for supporting the helical heating element with the processing tube 13 carried therein within the metal casing 12 and for insulating the same from the exterior Walls of the metal casing 12. Such means consists of a relatively large elongate tube 24 formed of a suitable material such as mullite which is concentric with the helical heating element 19 and surrounds the helical heating element. The tube 24 preferably is of such a size that the inside diameter or the diameter of the bore 26 within the tube is greater than the external diameter of the helical heating element 19 so that the helical heating element 19 can be spaced from the tube 24 as hereinafter described.

Means is disposed within the tube and engaging the helical heating element for supporting the heating element 19 so that it is spaced from the inner side wall of the tube 24 and consists of a plurality of circumferentially spaced rows of comb-like members 27 extending longitudinally of the tube 24 and of the heating element 19. The comb-like members 27 are formed in sections which have a length which is substantially less than the length of the helical heating element. The comb-like members are provided with a base portion 28 and with a plurality of parallel spaced teeth 29. The base portions of the comb-like members 27 are disposed in a plurality of circumferentially spaced recesses 31 provided in the tube 24 and extending longitudinally thereof. The recesses open inwardly in a radial direction into the bore 26 and they are adapted to carry the comb-like members 27 so that the teeth 29 face inwardly towards the center of the tube 24 in radial directions. The teeth 29 are disposed between the loops or coils 21 of the helical heating element 19 in the spaces 22 provided between the helical elements. The helical elements are seated in the recesses 30 provided between the spaced teeth 29 of the comb-like members 27. As can be seen particularly from FIGURE 3, the length of the teeth 29 are such that they extend beyond the inside diameter of the heating element so that the teeth 29 serve to support the processing tube 13 so that it is spaced from the helical heating element 19. The comblike spacing members 27 are formed of any suitable insulating material such as mullite which will withstand relatively high temperatures as, for example, 1400" C.

As can be seen particularly from FIGURE 1, a plurality of the comb-like spacing members 27 are provided in each of the slots 31. They are positioned next to each 'other"in"tlre"slot and extend over the ends of'the heating element 19. By way of example, the comb-like spacing members 27 could be 5 inches in length with the heating element being 45 inches in length so that each of the slots would have nine of the comb-like members. At least three circumferentially spaced rows, and preferably as many as five rows as shown in FIGURE 4, are provided for supporting the helical heating element 19.

Means is provided for supporting the large tube 24 within the metal casing 12 and which will serve as suitable insulation and consists of fire brick 36, or other material, to provide a rectangular hole 37 extending longitudinally of the metal casing 12 and in which the tube 24 is carried. A short tubular member 39 is provided at each end of the heating element 19 and engages the ends of the comb-like spacing members 27, as shown in FIG URE 2, and serves to longitudinally position the heating element 19 within the large tube 24. The tubular members 39 are provided with bores 41 through which the processing tube or member 13 extends. The tubular members 39 terminate flush with the ends of the large tube 24. The large tube 24 is mounted in an annular recess 42 provided in rings 43 carried by the fire brick 36 and by the metal casing 12. The rings 43 are provided with holes 44 through which the processing tube or member 13 extends.

Means is provided for supplying electrical energy to the helical heating element 19 and consists of terminals 46 which are mounted in the casing 12 and which are connected as shown to conducting bars 47 connected to the heating element 19. A control system is provided for supplying energy to the heating element 19. Such a control system is described in Patent No. 3,291,969. As disclosed in US. Letters Patent 3,299,196, such a control system includes a thermocouple assembly for sensing temperatures within the processing zone 17. As disclosed in Patent No. 3,299,196, such a thermocouple sensing assembly can be mounted so that the wires from the thermocouple sensing assembly extend out of one end of the casing 12. Alternatively, if desired, side mounting can be provided for the thermocouple assembly as. shown in FIGURES 4, S and 6 of the drawings.

As shown in FIGURES 4, 5 and 6 of the drawings, a thermocouple assembly 51 includes a mullite tube 52 which is mounted in the side wall of the casing 12 and in the fire brick 36. The tube 52 has its inner end mounted in a sleeve 55. The sleeve 55 is provided with a pair of spaced ears 55a which extend into the space between adjacent coils of the heating element 19. A smaller removable mullite tube 53 is removably mounted in the mullite tube 52 and is provided with a plurality of holes (not shown) through which wires (not shown) are connected to form a thermocouple junction 54. The mullite tube 53 is provided with a pair of spaced collars 56 and 57. The tube 53 is held in place in the sleeve 55 by yieldable spring means in the form of a U-shaped spring 58 carrieds by posts 59 and engaging the collar 57 to hold the tube 53 in place. Wires 61 extend from the collar 56 and are connected to a terminal block 62 where they are connected by wires 63 to the remainder of the control system for the diffusion furnace. It can be seen that the tube 53 with the thermocouple therein can be readily removed from the larger outer tube 52. The arrangement is such that the thermocouple junction 54 always has a fixed position relative to the heating element 19. If expan sion or contraction of the diffusion furnace occurs, the tube 52 will shift relative to the fire brick 36 and the casing 12. This, however, will not shift the position of the thermocouple junction 54 because the tube 52 is yieldably held in place against the sleeve 55 by the spring 58.

In assembling the diffusion furnace, the helical heating element 19 is taken and the comb-like members 27 are positioned on the heating element in five circumferentially spaced rows with a spacing so they will fit in the slots 31 in the large outer tube 24. This is accomplished by ushin the teeth 29 into the spaces 22 between theloe 's or coils of the heating element 19. The comb-like members 27 are then held in place by the frictional engagement between the comb-like members and the heating element. As soon as the fiverows have been completed, the heating elementwith the comb-likemembers mounted thereon is slidinto the outer tube 24 by moving the comb-like members into the slots 31 provided in the tube 24. After this has been accomplished, the two small end tubes 41 are mounted upon the'processing tube or member 13 to longitudinally position the rows of comb-like members and the heating element carried thereby within the metal casing 12. Thereafter, the rings 43 are put in place and the end walls of the casing are installed.

From the foregoing, it can be seen that the diffusion furnace can be readily assembled and that it is unnecessary to place individual spacers between each of the turns of the helical heating element. Rather, the comb-like members can be rapidly inserted and the helical heating element with the comb-like members mounted in the tube 24 with a substantial saving in labor.

Another advantage of the diffusion furnace is that the comb-like members serve to space the helical heating element away from the side wall of the tube 24 so that the helical heating element can heat up more rapidly when energy is applied to the same. This is because since the heating element is not in contact with the tube 24, the tube 24 cannot act as a heat sink to take heat away from the heating element. To speed up the heating of the heating element, it may be desirable to coat the inside of the tube 24 with a reflecting surface by glazing the same.

In the drawing, the helical heating element 19 has been shown as having a constant pitch. It should be appreciated that, if desired, portions of the heating coil or element could have a variable pitch as, for example, 'by having a relatively wide pitch at the center and then gradually increasing the pitch towards the ends of the heating element to provide a longer uniform temperature processing zone within the processing tube 13.

Another embodiment of the dilfusion furnace is shown in FIGURES 7 and 8. It is substantially identical to the embodiments hereinbefore described with the exception that additional tubular means is provided inside the heating element to help maintain a uniform temperature for a boat to be utilized in the processing zone of the processing tube 13. Thus, as shown in FIGURE 7, a tube 66 of high conductivity material, such as silicon carbide, beryllia or graphite is provided. Because graphite may produce a dust, it is desirable to cover it with a suitable material such as quartz. The tube 66 would have a length which is long as the flat zone desired for the diffusion furnace so that when a boat, such as a boat 67 indicated in broken lines in FIGURE 7, be inserted therein, would be maintained at a uniform temperature with no high or low temperature spots. This also would ensure that the boat 67 will come up to temperature evenly after it is inserted within the tube.

Two additional tubes 68 and 69 are provided which are formed of a suitable material such as mullite and have a conductivity which is substantially less than the conductivity of the tube 66. These tubes 68 and 69 are provided with annular recesses 71 and 72 which are adapted to receive the ends of the tube 66 as shown in FIGURE 7. The two outer insulating tubes 68 and 69 serve to prevent the loss of heat outwardly from the central tube' 66. The outer tubes thus, in effect, serve to extend the flat region or uniform temperature region within the tube 66. Operation of this embodiment of the invention is substantially similar to that hereinbefore described.

Still another embodiment of the invention is shown in FIGURE 8 in which, in place of the tube 66, there has been provided a curved plate 76 formed of a material having a high thermal conductivity and upon which the processing tube 13 is adapted to rest. The curved or arcuate plate 76 is disposed in the recesses 71 and 72 of end tubes 68 and 69 formed of an insulating material to thermally insulate the ends of the arcuate plate 76 and to also provide an elongated fiat or uni-form temperature zone within the processing tube 13. It should be appreciated that, if desired, the end tubes 68 and 69 can be omitted entirely and the curved plate 76 can be.

supported by extensions of the comb-like members 27 extending through the helical heating elements and serving to carry the arcuate plate 76.

In each of the embodiments shown in FIGURES 7 and 8, end mounted or side mounted thermocouple assemblies of the type hereinbefore described can be utilized.

It is apparent from the foregoing that there has been provided a diffusion furnace which can be readily assembled with a great saving in labor cost. In addition, it can be seen that the heating element is positioned in such a manner that it can be heated up very rapidly without the necessity of heating all of the surrounding supporting structure, thus making it possible to bring the processing zone in the processing tube up much more rapidly to the desired operating diffusing temperature.

I claim:

1. In a diffusion furnace, a substantially hollow processing member having a processing zone therein, a helical heating element surrounding the processing member and extending longitudinally thereof, said helical heating element being formed of a plurality of loops with spaces therebetween, and means supporting the heating element, said means for supporting the heating element including a tube generally concentric with the heating element including a plurality of comb-like spacer members, each of said comb-like spacer members being formed with a plurality of teeth disposed in the spaces between loops of the heating element, said comb-like spacing members being spaced circumferentially about the heating element to support the heating element within the tube and to space the heating element from the inner wall of the tube.

2. A diffusion furnace as in claim 1 wherein said tube is provided with circumferentially spaced slots extending axially of the tube and opening inwardly into the tube.

3. A diffusion furnace as in claim 2 wherein said comb-like members are arranged in rows, and said rows of comb-like members being disposed in said slots and being held in predetermined positions in said slots.

4. A diffusion furnace as in claim 1 together with a member formed of high thermal conductivity material disposed within said tube generally intermediate the ends of the processing tube and serving to support the processing tube and to conduct heat to the processing tube.

5. A diffusion furnace as in claim 4 together with addition-a1 members formed of a material having good insulating properties and engaging the ends of said member having good thermal conductivity for insulating the ends of the member of high thermal conductivity and for extending the flat zone within the diffusion furnace.

6. A diffusion furnace as in claim 4 wherein said memher having high thermal conductivity is a cylinder and encircles the processing member.

7. A diffusion furnace as in claim 6 wherein said members formed of a good insulating material, are also cylindrical in form and encircle the processing member.

8. A diffusion furnace as in claim 1 wherein the teeth extend into the interior of the heating element beyond the inside diameter of the heating element to support and space the processing member from the heating element.

9. A diffusion furnace, a substantially hollow processing member having a processing zone therein, helical heating elements surrounding the processing member extending longitudinally thereof, means for supporting the heating element and means for sensing the temperature of the heating element, said means for sensing the temperature of the heating element including a tubular element formed of insulating material slidably mounted in the means for supporting the heating element, means forming a thermocouple junction carried by the tubular element and serving to hold the thermocouple junction in a position adjacent the heating element, and yieldable means carried by the means for supporting the heating element for yieldably urging the tubular element in a direction toward the heating element so that the thermocouple junction is retained in a relatively fixed position with respect to the heating element regardless of contraction and expansion of the diffusion furnace.

10. A furnace as in claim 9 wherein said helical heating element is provided with a plurality of loops with spaces therebetween and wherein said means for sensing the temperature of the heating element includes a sleeve having a pair of cars disposed in the space between adjacent loops of the helical heating element, said sleeve having a recess therein adapted to receive one end of the tubular element, said sleeve being provided with an opening through which the means forming a thermocouple extends.

11. A diffusion furnace as in claim 9 together with an additional tubular element surrounding said first named tubular element and being disposed in said sleeve and permitting removal of said first named tubular element.

12. A diffusion furnace as in claim 11 wherein said additional tubular element has a collar formed thereon which engages the first named tubular element and wherein said yieldable means engages said collar to urge said additional tubular element into engagement with said first named tubular element.

References Cited UNITED STATES PATENTS 2,611,790 9/1952 Koch l320 2,661,385 12/1953 Lincoln et a1. 1320 2,831,909 4/1958 Seifert 13-20 X 2,895,433 7/1959 Michaelis 13 22 X 3,083,445 4/1963 Hill l322 X 3,128,326 4/1964 Hintenberger 1322 X 3,140,335 7/1964 Muth et al. 13-20 2,010,768 8/1935 Morgan 2l9-54O BERNARD A. GILHEANY, Primary Examiner. H. B. GILSO'N, Assistant Examiner. 

